Why it is a taboo to marry your sibling

Some countries even criminalise the concept of incest, with constitutional laws to prosecute offenders.

But why is it such an issue for siblings to engage in sexual relationships?

Offspring of incestuous couples are more likely to suffer birth defects and genetic disorders, especially due to recessive inheritance. Recessive inheritance means both genes in a pair must be defective to cause the disease, hence, people with one defective gene will not have the disease.

However, when close blood relatives have children, both parents share very similar DNA and genes, making it more likely for their children to inherit these recessive genetic disorders.

So, even when the parents do not have the disorder because they have singular genes, it can easily become a disorder-causing pair in their offspring.

This does not only apply to sibling couples' offspring — procreation between first cousins or other close relatives may also cause recessive genetic disorders, as they may share very similar genes.

Aside from potential medically disastrous offspring, incest is considered a form of sexual immorality and sin in many religions. Islam generally forbids incest with few exceptions. The Quran refers to incestuous relationships as immoral and hateful towards Allah.

The Bible also prohibits any form of sexual relation among close relatives, even if they are not directly related by blood.

Sexual relationship between siblings or close relatives is a tradition that society has frowned upon for generations. Whether a marriage or sexual fling, it just feels out of place and socially awkward. People who commit incest may be stigmatised or excommunicated.

While relationships between close relatives are illegal, there are exceptions in some countries. The law may allow you to marry a relative if the percentage of DNA shared (consanguinity) between the two parties is less than 25%.

Hence, second cousins or other distant relatives who share low DNA similarities may be free to marry.